The lock of the invention is of the type which comprises two members adapted for automatic interlocking engagement on the introduction of one member into a cavity of the other member. The first member is a rod-shaped body with a tapered insertion end and with an abruptly restricted cross-section area slightly rearward of the tapered section so as to form a shoulder, and the second member is a casing with a bore in which is provided a tubular sleeve mounted with limited or no mobility in the axial direction. The sleeve is slit at the end located innermost in the casing and its sectional opening at the slit end is adapted to permit the shoulder of the rod-shaped body to be passed through the tubular sleeve only as a result of the elasticity provided by the slit in the end of the sleeve.
Single engagement locks of this type are known from U.S. Pat. Nos. 3,730,578 and 1,553,188. In the single engagement lock disclosed by U.S. Pat. No. 3,730,578 the second member is a block with throughgoing openings, the diameter of which is smaller at the end where the rod-shaped body is introduced than at the opposite end, which is closed by an inserted sleeve provided with a bore facing the interior of the block and of the same diameter as the smallest diameter of the throughgoing opening. The sleeve extends sufficiently far into the block to form within the block an annular groove for a lock ring which will be expanded on the introduction of the rod-shaped body into the block until the rod-shaped body has penetrated so that the shoulder thereof has passed the lock ring, which then prevents the rod-shaped body from being withdrawn.
Such a single engagement lock is used partly as a padlock but specifically as a seal, for instance, on containers for the transport of goods. The opening of the lock requires the use of force and the fact that the lock is intact indicates that no unauthorized person has had access to the interior of the container.
However, it was found that a lock of this known type could be opened and relocked without leaving any direct signs of the use of force, and thus the security, that the contents on receipt were the same as dispatched, would be lost. Furthermore, the interlocking of the two members was not very secure because the known lock could be opened by impact or by means of a crowbar.
In the single engagement lock disclosed in U.S. Pat. No. 1,553,188 the aforesaid lock ring is replaced by a tubular sleeve open at both ends. Theoretically this results in an improved interlocking connection between the two members of the lock, but this lock can also be broken by a relatively simple operation, since the block member consists of procelain or plastic material, though it could probably not be relocked without leaving signs of the opening.